from pandas.core.common import flatten

l1 = [[1, 2, 3], [4, 5, 6, 7], [8, 9, 10, 11]]
l2 = [[[1, 2], 3, [4, 5, [6, 7]]], [[8, 9], [10, 11]]]
l3 = [["a", "b"], ["c", "d"], "e"]

print(list(flatten(l1)))
print(list(flatten(l2)))
print(list(flatten(l3)))

